1. Field of the Invention
The present invention relates to a base structure of an adhesive tape to be used for precision-machining of an optical structural element such as a lens, a semiconductor structural element such as a wafer, or the like. The present invention also relates to a method of manufacturing the above base structure and an adhesive tape having the base structure on which an adhesive layer is formed.
2. Description of the Prior Art
Heretofore, adhesive tapes have been used in the fields of optical industry, semiconductor industry, and so on, for manufacturing structural elements that require precision-machining. In the following description, as a concrete example of the conventional one, we will explain an adhesive tape used in the field of manufacturing a semiconductor wafer.
Generally, the adhesive tape can be prepared by the process including the steps of: melting a thermoplastic resin; forming a film-shaped base structure as a base sheet by subjecting the melted thermoplastic resin into the process of using a T-die, a blow-extrusion, a calender, or the like; and applying an adhesive agent on at least one surface of the base sheet. The adhesive tape thus obtained is typically used for holding a semiconductor wafer in place during the step of grinding the wafer's back and also the step of dicing and isolating the wafer.
At first, we will explain the example of using the adhesive tape in the process of grinding a back side of a semiconductor wafer that has a front side on which a predetermined pattern is formed.
At the time of grinding the back side of the wafer, the wafer's back is continuously rinsed by running pure water to remove scattered or fragmented remains (i.e., debris) and to reduce heat generated by the grinding. In this case, therefore, an adhesive tape is applied on the wafer's front to protect the pattern formed thereon from the grinding.
At the time of performing the step of dicing the semiconductor wafer into pieces, an adhesive tape is applied on the wafer's back, so that the wafer on which the adhesive tape is being adhered is subjected into each of the steps of dicing, washing, drying, expanding, pick-upping, and mounting, successively.
These steps requires that the adhesive tape should have the properties of, for example without remaining any part of the adhesive agent on a wafer or a chip when the adhesive tape is removed after processing. For solving this problem, several improvements have been disclosed in patent documents including Japanese Patent Application Laying-open No. 62-153376 (1987) and Japanese Patent Application Publication No. 5-77284 (1993).
An adhesive sheet disclosed in Japanese Patent Application Laying-open No. 62-153376 (1987) comprises an adhesive containing urethane acrylate (m.w. 3,000.about.10,000) as a radiation-induced polymerized compound applied on a base material. Therefore, an adhesive strength of the adhesive sheet is sufficiently decreased by an effect of irradiating with ultraviolet rays or the like in the step of pick-upping after dicing the semiconductor wafer, as that the adhesive agent does not remain on the back of the wafer chip. On the other hand, an adhesive tape disclosed in Japanese Patent Application Publication No. 5-77284 (1993) comprises a water-swelling type adhesive agent as an adhesive layer applied on a surface of a base sheet. In this case, therefore, it becomes to possible to remove the adhesive agent attached on the wafer's surface by running water after the grinding process.
Each of the adhesive tapes described above uses the base material prepared as a film-shaped material by melting a thermoplastic resin such as vinyl chloride or polypropylene and subjecting the melted thermoplastic resin into the process of using a T-die, a blow-extrusion, a calender, or the like, so that the conventional adhesive tapes have the following disadvantages which should be resolved by the present invention. That is, several small projections are sparsely formed on a surface of the base sheet as a result of mixing undesired foreign particles, insoluble portions of the resin components, or the like into the resin at the time of forming the film-shaped base material. Each of those projections are in the shape of a fish's eye with a height of about 10 to 50 .mu.m (hereinafter the projection is referred as a fish-eye). These fish-eyes cause several disadvantages as described in the follows.
FIGS. 1 and 2 are schematic cross sectional views of a semiconductor wafer supported by a conventional adhesive tape. FIG. 1 illustrates the step of grinding a back side of the semiconductor wafer, while FIG. 2 illustrates the step of dicing the semiconductor wafer.
An adhesive tape 10 comprises a base sheet 11 and an adhesive layer 12 applied on the base sheet 11. As shown in the figures, the adhesive tape 10 is being adhered to a surface 14 (where a pattern is formed) of a semiconductor wafer 13. In the step of grinding, a back side of the semiconductor wafer 13 is subjected to a grinding machine (not shown). In this case, however, the back side's surface receives uneven pressure from the grinding machine because of the existence of the above fish-eyes thereon. That is, there is the pressure difference between a portion having a fish-eye and an intact portion of the back side and it results in cracks to be extended from the fish-eye on the semiconductor wafer. In the step of dicing the semiconductor wafer 13, furthermore, the adhesive tape 10 is being adhered to the back side of the semiconductor wafer to be diced into chips from the front side thereof. During the dicing, uniform pressure is being applied from the back to the front of the wafer. In this case, however, there is the pressure difference between a portion having a fish-eye and an intact portion of the back side and it results in flying a chip off when the wafer is diced.
For solving these disadvantages, there is the attempt to eliminate the fish-eyes forcefully by pressing the base material by a mold component in the conventional process of preparing the adhesive tape. However, this attempt is not a fundamental problem-solving technique and requires an additional step, so that it leads to confuse its manufacturing process and to increase a production cost of the adhesive tape.